gadgets.""No science," corrected Coghlan, busy at the wall. "They achieved some results by accident. Then they repeated all the things that had preceded the unexpected result, and never knew or cared which particular one produced the result they wanted. Tempering swords, for example."Duval interposed: "The Byzantine Empire imported its finer swords.""Yes," agreed Coghlan. "Religion wouldn't let them use the best process for tempering steel.""Religion?" protested Mannard. "What did that have to do with tempering swords?""Magic," said Coghlan. "The best temper was achieved by heating a sword white-hot and plunging it into the body of a slave or a prisoner of war. It was probably discovered when somebody wanted to take a particularly fancy revenge. But it worked.""Nonsense!" snapped Mannard."Some few cutlers use essentially the same process now," said Coghlan, absorbed in removing a last bit of plaster. "It's a combination of salt and nitrogenous quenching. Human blood is salt. Steel tempers better in salt water than in fresh. The ancients found that human blood gave a good temper. They didn't think scientifically and try salt water. And the steel gets a better surface-hardening still, if it's quenched in the presence of nitrogenous matter--like human flesh. Cutlers who use the process now soak scrap leather in salt water and plunge a white-hot blade in that. Technically, it's the same thing as stabbing a slave--and cheaper. But the ancients didn't think through to scrap leather and salt water. They stuck to good old-fashioned magic tempering--which worked."He stood back. He brushed plaster dust off his fingers."That's all we can do without more apparatus. Now--"He picked up the alnico magnet and moved it across all the cleared space. An oblong pattern of silveriness appeared at the nearest part of the wet place to the magnet. It followed the magnet to the edge, and ran abruptly off into nothingness as the magnet passed an invisible boundary."At a guess," said Coghlan thoughtfully, "this is the ghost, if you want to call it that, of what the ancients thought was a magic mirror--to look into the future with. Right, Duval?"Duval said tensely:"It is true that all through the middle ages alchemists wrote of and labored to make magic mirrors, as you say.""Maybe this one started the legend," said Coghlan."The flashlight battery's getting weak--" Ghalil's voice from the darkness."We need better light and more apparatus," said Coghlan. "I doubt if we can do any more before morning."His manner was matter-of-fact, but inside he felt oddly numb. His thumb stung a little. The cut had been irritated by plaster-dust and by the soot that got into it when Ghalil took a fresh thumbprint to show Mannard. In the last analysis, he'd cut his thumb investigating the ghost 

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